Electric valve control system



Oct. 10, 1939. H.WINOGRAD ELECTRIC VALVE CONTROL SYSTEM Filed Dec. 29, 1937 2 Sheets-Sheet l Oct. 10, 1939.

H. WINQGRAD 7 W810 VALVE CONTROL SYSTEM 2 Sheets-Sheet 2 Filed Dec. 29, 1937 Patented Oct. 10, 1939 UNITED STATES PATENT OFFICE ELECTRIC VALVE CONTROL SYSTEM Application December 29, 1937, Serial No. 182,196

4 Claims.

This invention relates in general to improvements in electric valve control systems, and more particularly to means for causing a plurality of electric valves connected in parallel circuits to 5 carry current simultaneously during substantially coextensive periods.

Electric valves are frequently utilized in connection with inductive apparatus to form alternating current rectifying systems or other sysl0 tems operable to transmit electric current between circuits having different electrical characteristics. In such systems each valve is connected with at least one alternating current circuit which may be a supply circuit, an output circuit or an inter- 16 mediate circuit, and the current through the valve is a pulsating current. It is then generally desirable or even necessary to control the initiation of the successive periods of flow of pulsating current through each valve by means of a control elec- 20 trode, and to obtain this result the control electrode of each valve is caused to receive suitable potentials which are preferably obtained from the alternating current circuit connected With the valve.

When a system is intended to transmit large amounts of energy, the current flowing through some or all branches of the system may be of higher magnitude than the maximum rated current of the valves which it is intended to utilize or even of any valve available. It is then generally convenient to replace each valve by two or more valves connected in parallel circuits and designed to function simultaneously during substantially coextensive periods to carry currents of magnitudes substantially in a predetermined ratio. The operation of the valves, however, generally causes a distortion of the voltage of the alternating current circuit associated therewith, and the control potentials impressed therefrom on the control electrodes of the valves are likewise distorted from the sinusoidal wave form. This distortion may become sufficiently severe to cause the valves connected in parallel to carry currents of magnitude departing materially from the desired ratio or even to cause some of the valves to become entirely inoperative.

The foregoing disadvantage may be obviated by insuring in any manner that the alternating current control potentials impressed on the control electrodes be of substantially sinusoidal Wave form, or at least be free of such distortion as may interfere with the desired control. This result is preferably obtained by means of a filter or of filters of the so-called low-pass type inserted in the connections, between the control electrodes of the valves and the associated alternating current circuit. The filters may also be designed to cause the control potentials to have suitable wave form departing from the sinusoidal wave form.

It is therefore one of the objects of the present 5 invention to provide a control system for controlling a translating system comprising a plurality of valves connected in parallel circuits to cause the valves to carry currents of magnitudes substantially in a predetermined ratio. 10

Another object of the present invention is to provide a control system for controlling a translating system comprising a plurality of valves connected in parallel circuits to cause the valves to carry pulsating currents initiated substantially 15 simultaneously in the parallel valves.

Objects and advantages other than those above described will be apparent from the following description, when read in connection with the accompanying drawings, in which: 20

Fig. 1 diagrammatically illustrates one embodiment of the present invention applied to the control of an alternating current rectifying and direct current inverting system employing electric valves connected in pairs in parallel circuits; 25

Fig. 2 is a diagram of some of the currents and voltages involved in the circuits of the embodiment illustrated in Fig. 1;

Fig. 3 is a curve of the effectiveness of the filter of the embodiment illustrated in Fig. 1; 30

Fig. 4 is a diagram illustrating a modification of the filter of the embodiment illustrated in Fig. 1; and

Fig. 5 is a curve of the effectiveness of the filter of the embodiment illustrated in Fig. 4. 35

Referring more particularly to the drawings by characters of reference, reference numeral 6 designates a three phase alternating current line or circuit connected with a dynamo-electric machine which is represented as being of the syno chronous type operable either as a generator or as a motor. Regardless of the mode of operation of machine I, it may be assumed that the induced voltage thereof is a substantially sinusoidal voltage of a predetermined fundamental frequency, 45 and that the no load terminal voltage of machine I is also a substantially sinusoidal voltage. When energy is being transmitted between machine 1 and circuit 6, however, the terminal voltage of machine I may be distorted from the sinusoidal 5 Wave form as a result of the flow of non-sinusoidal current through the windings of the machine, if such windings have appreciable reactanoe.

Circuit 6 is to be connected with a direct current line or circuit through a converting system comprising a transformer 9 having a plurality of windings M, i2, it, M, iii. Winding ll comprises a plurality of phase portions connected with circuit 6 at a point which will be assumed to be connected with machine 'l through a portion of circuit 6 having a material impedance. Of the different components of such impedance, the resistance component may generally be neglected, but the series inductance component is frequently important, particularly in high voltage circuits of considerable length, In circuits comprising portions of insulated cable, the impedance of the circuit may also comprise an appreciable shunt capacitance component. The presence of such impedance components is indicated on the drawings by assuming the inductive reactance of the portion of circuit 5 considered to reside in a bank of fictitious reactors it and the capacitance of such portion to reside in two groups of fictitious capacitors ll as is usual in practice. Reactors l 6 may also be assumed to include an appropriate amount of reactance equivalent to the reactance of machine l, and the terminal voltage of machine ll may then be assumed to be substantially sinusoidal under all operating conditions.

Each of windings l2, id, id and it comprises a plurality of phase portions connected in star to provide a neutral point. Windings l2 and i i are similar to each other and jointly provide a plurality of circuits connected in parallel in pairs to receive currents initiated substantially simultaneously in the two circuits. Windings i3 and iii are likewise similar to each other to jointly provide a plurality of circuits connected in parallel in pairs. These four windings are connected with one conductor of circuit 8 through connections preferably comprising a pair of inter-- phase transformers l3, l9 and a reactor 2!. Although transformer 9 is illustrated as being of the three phase to six phase type, it will be understood that the present invention is also applicable to the control of systems utilizing transformers of any number of phases, including single phase transformers. Transformer 9 is unavoidably endowed with a certain amount of reactance consisting principally of leakage inductive reactance and which may be assumed to reside in a three phase bank of fictitious reactors 22 connecting the different phase portions of winding i i with circuit 6.

A plurality of electric valves generally designated by are severally connected with the terminals of windings i2 to M5 and with the second conductor of circuit 8. The valves are thus connected with circuit 6 at a point connected with machine l through a portion of circuit 5 having a material impedance assumed to reside in members it, ill and are also connected with circuit 8 for the flow of current between circuits 6 and B. Each one of the valves connected with one of the terminals of windings l2 and a corresponding valve connected with one of the terminals of windings i i and i5 are connected in parallel circuits, and the two valves of each parallel connected pair are intended to carry currents of magnitudes substantial y in a predetermined ratio initiated substantially simultaneously therethrough. In general, all the valves are of a uniform design and it is accordingly desired that all the valves carry currents of substantially equal magnitude.

Valves 23 may be of any known controllable type and are preferably of the discontinuously controllable or vapor type, each having a plurality of electrodes including an anode and a cathode. The anodes such as 26, 25, 26, 21 of the valves may be severally arranged in physically distinct structures, but are preferably arranged Within a common casing, and the cathodes of the valves may then be combined into a common cathode structure 28 preferably comprising a pool of mercury. Suitable means (not shown) are provided for bringing cathode 28 into electron emitting condition and for maintaining the cathode in such condition, as is well known.

For the purpose of controlling the operation of valves 23, each valve is provided with suitable means for controlling the conductivity thereof, such as a control electrode. The control electrodes such as 29, iii are diagrammatically represented as grids on the drawings, but other types of control electrodes, such as cathode spot igniting devices, may also be utilized when the valves are provided with separate cathodes. Valves 23 being of the discontinuously controllable type, each control electrode, as is well known, prevents the flow of current through the associated anode when the control electrode is at a negative potential with respect to the so-called critical potential of the valve, and the control electrode releases this flow of current upon being brought to a positive potential with respect to the critical potential. Although the critical. potential of a control electrode is frequently considered as being equal to the cathode potential, it varies slightly in dependence upon. the value of the voltage between the corresponding anode and the cathode. In addition, in valves of the mercury pool cathode type, the critical potential varies from one instant to another, and differs in value for different valves, even if such valves are enclosed in a common casing. In particular, the critical potentials of the control electrodes. such 29, SI, of two valves connected in parallel circuits will diifer in value by an amount which aifects the operation of the valves, even if such amount is small relatively to the value of the potentials impressed on the control electrodes.

To cause substantially simultaneous initiation of the currents flowing through the two valves of each pair of valves 23 and the sequential initiation of the current flow through the different pairs of valves, the control electrodes are associated with a control system comprising suitable sources of control voltages connected therewith. The sources bring the control electrodes to alternately positive and negative potentials with respect to the cathode potential taken as datum. The control sources comprise a source of substantially sinusoidal alternating voltage of fundamental frequency. for which machine 7 is preferably utilized, and a source of unidirectional voltage which be circuit il or a suitable direct current generator 32.

Each pair of control electrodes, such. as 25!, 3|, controlling the flow of current through a pair of parallel anodes, such as 26, 2i, is preferably con nected with machine l through current limiting resistors 33, and through a common connection comprising one of the phase portions of the secondary winding 35 of a control transformer 31 having a primary winding connected with machine l. Transformer is provided with a core 49 which may be of any type so dimensioned as not to be subjected to magnetic saturation, or else core ll may be of the plural single phase type or of the shell type and saturated to cause the voltages appearing in the phase portions of winding 38 to have a wave form presenting a steep front. Winding 36 is connected in star to provide a neutral point connected with the tap of a voltage divider 39 connected with the terminals of generator 32 and with cathode 28, whereby the control electrodes are also connected with circuit 8. Transformer 31 may be of the usual type in which the secondary phase voltages are in phase with the corresponding primary phase voltages or may be of the variably phase shifting type. Suitable filtering means, such as a capacitor 42, is preferably connected with cathode 28 and with the neutral point of winding 36 to render the voltage impressed therebetween from voltage divider 39 substantially uniform. As is usually found expedient in practice, winding 38 is connected with machine 1 by connecting winding 38 with circuit 6 at substantially the same point as winding II, or at least at a point of circuit 6 connected with machine I through substantially the entire portion of circuit 5 having the impedance represented by elements I6 and I].

As will appear hereinafter the flow of current between machine 1 and winding ll through this impedance causes the potentials impressed from machine 1 on the control electrodes of valves 23 to comprise alternating components of frequencies other than the fundamental operating frequency of circuit 6. The connections between circuit 6 and winding 38 also frequently comprise a plurality of impedance elements such as resistors 43 provided to permit short circuiting winding 38' to cause interruption of the flow of current through valves 23, which resistors may contribute to the distortion of the voltages impressed from circuit 6 on winding 38. Short circuiting of winding 38 may be effected in response to the magnitude of the flow of current through valves 23 by means of a relay 44 energized from a current transformer 46 inserted in one of the connections of winding II with circuit 6. To enable relay 44 to reset while maintaining valves 23 inoperative, relay 44 is associated with the second short circuiting relay 4'! having the coil thereof connected across one of resistors 43 through another resistor 48. Relay 4! may in turn be reset manually by short circuiting the coil thereof by means of a switch 49.

To insure the impression of voltages of sinusoidal or other predetermined wave form across the terminals of winding 38, regardless of the Wave form of the voltage of circuit 6, the connections between circuit 6 and winding 38 include electric filter means of a suitable type such as the so-called low-pass type. Transformer 3'! being assumed of the polyphase type, the filter associated therewith is likewise of the polyphase type and comprises a plurality of reactors 5| serially inserted in the connections of winding 38 and a plurality of capacitors 52 connected across the terminals of winding 38. Reactors 5| are tuned with capacitors 52 for a frequency which may be selected at different values materially different from the fundamental operating frequency of circuit 6.

The operation of the system will be considered under the assumption that circuit 6 is a supply circuit receiving alternating current from machine and that circuit 8 is an output circuit supplying rectified current to suitable load devices (not shown). The flow of energy is accordingly from circuit 6 to circuit 8 but it will be understood that the system is equally adapted for the flow of energy from circuit 8 to circuit 6 by suitable adjustment of the control elements thereof. During rectifying operation of the converting system, the voltages impressed from circuit 6 on the phase portions of winding ll induce corresponding voltages in the associated phase portions of windings I2, l3, l4 and I5 whereby the anodes of valves 23 are sequentially brought to a positive potential with respect to the potential of cathode 28. When an anode has thus assumed a positive potential, the flow of current thcrethrough is prevented as long as the associated control electrode remains at a potential more negative than the critical potential. Upon receipt by this control electrode of a potential more positive than the critical potential from the joint action of winding 36 and of generator 32, the flow of current through the associated anode is released and continues until the transfer thereof to another anode. The successive anode current impulses thus produced combine at cathode 28 to form a flow of substantially uniform direct current, as is well known in the art.

The foregoing operation may be followed in greater detail with the aid of the curves of Fig. 2 which are drawn assuming that the ratio of transformer 9 is unity and neglecting the effects of the resistance of circuit 6, of the resistance of the windings of transformer 9, and of the arc drop in valves 23. Voltage divider 39 is as-- sumed to be adjusted to delay the conductive periods of valves 23 by approximately 20 degrees. The assumed sinusoidal terminal voltage of one of the phases of machine I may be represented by a sinusoidal curve 53. As is well known, windings l3 and I5 and the associated valves operate substantially independently of windings I2 and M as the result of the action of interphase transformers I8, IS. The voltage appearing between the neutral points of windings l3 and I5 and cathode 28 is then represented by a curve 54 consisting of portions of curve 53 and of two similar sinusoidal curves separated by portions of other sinusoidal curves corresponding to the so-called period of overlap between anode conductive periods. It will be understood that as a result of the action of reactor 2!, the flow of current through cathode 28 is substantially uniform, and the current through anodes 24 and 25, for example, may be represented by a fiat topped curve 56 while the current through anodes 26 and 2'! may be represented by a similar curve 51.

The transfer of the flow of current between anodes 24, 25 and 26, 2'! is opposed by the inductive reactance of circuit 6 and of transformer 9 and such transfer accordingly takes place gradually over a period during which all four anodes are carrying current, the so-called period of overlap. During this period two of the phase portions of each of windings l3 and 15 are operating momentarily in parallel at a common voltage equal to the average voltage of the phases of machine I inductively connected therewith. The difference between the phase voltages of machine 1 and of windings !3, I 5 may be considered as being, in one phase, the inductive voltage rise caused in the corresponding reactors l6 and 22 by the fall of the currents through anodes 24, 25 from the full value thereof to zero. In another phase, such difference is the effective voltage drop caused in the corresponding reactors l 6 and 22 by the rise of the currents through anodes 25, 21 from zero to the full value thereof. The voltage of one phase portion of each of windings l3 and I5 is then represented by a curve 58 differing from the sinusoidal curve 53 by periodic indentations corresponding to the periods of overlap. The effect of capacitors I1 is herein neglected to simplify the explanation of the operation of the system and to render the drawings clearer, but it will be understood that further distortion of the secondary phase voltages of transformer 8 is caused by the capacitors.

windings i2, l i operate similarly to winding i3, it; to deliver current under a voltage which may be represented by a curve (not shown) similar to curve but displaced by 60 degrees with respect thereto. As is well known, the voltage of circuit 6 is then the average of the output voltages of all windings l2 to 55, which is represented by curve 59. The inductive voltages due to the current changes during the periods of overlap may be divided between reactors iii and 22 in proportion to the inductances thereof and by adding to the phase voltages of machine l, represented by curves such as 53, the inductive voltages in the corresponding reactors to, the terminal voltages of winding ii may be obtained. Two of these voltages are represented by curves 6i and 62.

For the purpose of rendering more apparent the distinctions between the operations of the system illustrated in Fig. 1 and of the systems of the art, let it be assumed at first that filter 5!, 52 be removed. Winding 38, which is connected substantially at the terminals of winding H, is then subjected to the distorted voltages represented by curves 5!, 62 which may be further distorted by the flow of non-sinusoidal current from circuit to winding 38 through resistors 53. Because of the connection of winding 38 in star, each phase voltage thereof is obtained by combining two of the terminal voltages thereof. One of such phase voltages is represented by curve 53 obtained by difference from curves 6! and 82. Curve also represents the potential component impressed from winding 36 on control electrodes 29 and 3!. The potential of these control electrodes with respect to the potential of cathode 28 is then represented by the same curve 63 read with respect to a line 6 of positive ordinate equal to the negative voltage impressed from voltage divider 39 between cathode 253 and the neutral point of winding 36. To simplify the drawings, it may be assumed that the critical potential of control electrode 29 is constant and represented by a line 66, while the critical potential of control electrode 3! is more positive and represented by another line 61.

Dining each voltage cycle of circuit 6, while current is carried by at least one of anodes 24 and 25, control electrodes 29 and 3| both reach the potential of line 5 at a time represented by a point A, whereupon control electrode 29 releases the flow of current through anode 26. As a result thereof, a period of overlap is established between at least one of anodes 24, 25 and anode 2'6, and the potential impressed on control electrodes 29 and 3| drops to the value represented by point B of curve (53. Control electrode 29 is then, however, without further action on the operation of anode 26, which continues to carry current as represented by curve 57!. Control electrode (ti, on the contrary, not having reached the critical potential thereof, continues to prevent the fiow of current through anode 21 until the end of the period of overlap when control electrode 3.! reaches the critical potential thereof at point C. At this moment anode 25 already carries the full current intended for both anodes 26 and 21. This current is then substantially constant and therefore no longer produces any inductive effects in circuit 6 and in transformer 9 which would otherwise tend to raise the potential of anode 21 to cause anode 21 to carry part of the current. As a result thereof anode 21 is then at a potential higher than the potential of anode 26 by only the resistance voltage drop produced in winding i 3 by the flow of current therethrough, which voltage drop is generally of very small value. Anode 2i accordingly remains inoperative or, at the most, carries a small fraction of the current to be supplied to circuit Si whereby anode 25 is overloaded, at least if valves 23 deliver more than one-half of the rated current thereof. The above sequence of operation is re eated sequentially in the different circuits associated with valves 23 with the result that the valves are inefficiently utilized.

It is to remedy this defect that filter 5], 52 is provided. Because the faulty operation of valves 23 results from. the sudden decrease at point A of the control electrode potential represented by curve it is necessary to filter out from the control electrode voltage the components thereof which jointly form the indentations of curve 63. As may be determined by analysis of curve 63, the disturbing components thereof are harmonics of the fundamental frequency, principally the fifth and the seventh harmonics, and other higher harmonies of orders 2n+1.

It may be thought that the above result would be most simply obtained by means of resonant filters serially inserted in the connections between circuit 6 and winding 38. The inherent characteristic of series resonant filters is to transmit current of substantially sinusoidal wave form and indeed if such current flows through a load having a constant impedance, the load voltage will also be sinusoidal. In the present instance however the load, which consists of transformer S? and the control electrode circuits supplied therefrom, is an auxiliary six phase rectifying system which is not of constant impedance as is evidenced by the fact that when a sinusoidal voltage is impressed on winding 38, the current drawn by this winding from circuit 6 is not sinusoidal. If a sinusoidal current were maintained through winding 88, the voltages appearing in windings 38 and 36 would therefore not be of the sinusoidal form but rather of the general type represented by curve $3. In addition, series resonant filters must be tuned to the fundamental operating frequency of the circuit connected therewith and become ineifecive if the fundamental frequency varies during operation as it frequently happens in small independent electric systems. nant filters are also subjected to voltages considerably higher than the operating voltages of the associated circuits and their cost is accordingly excessive.

On the contrary, a filter of the low pass type tends to cause the voltage impressed therethrough on the load to be substantially sinusoidal, or at least to be free of components of frequencies above a predetermined value, while the load current may adjust itself to the nature of the load.

This will be apparent from a consideration of Fig. 3, which diagrammatically illustrates the eifectiveness of filter 55, 52, i. e. the values of voltages of different frequencies which would be impressed upon winding 38 through the filter if voltages of unit value of different frequencies were present in circuit 6 at the terminals of winding M. It will be observed that all voltages of frequencies below a predetermined value DE are increased in magnitude by the filter. On the The capacitors utilized in series resocontrary, all voltages of frequencies higher than the value DE are reduced in value, and this reduction is greater at relatively high frequencies than at relatively low frequencies. Abscissa DF represents the resonant frequency for which filter 52 is tuned, which may be selected at any desired value.

It is thus possible to select a filter so tuned that the frequency represented by abscissa DE is below the frequency of the fifth harmonic to be eliminated from the voltages of winding 38. The fifth harmonic may thus be reduced to any desired degree, and the harmonics higher than the fifth are reduced to a still higher degree than the fifth harmonic. The filter will increase the voltage of fundamental frequency transmitted therethrough to winding 38, if the frequency represented by abscissa DE is above the fundamental frequency. While the filter need not be tuned exactly for any particular frequency, the resonant frequency DB of this filter should, however, not be equal to the fundamental frequency of circuit 6 as such adjustment would cause the voltage impressed on transformer 38 to become excessive.

If the frequency DE is below the frequencies of all harmonics of the fundamental frequency of circuit 6, the voltage impressed on winding 38 will be substantially sinusoidal and may be represented by a curve 68. This voltage is, however, shifted in phase with respect to the voltage of circuit 6 so that to obtain the voltage of circuit 8 represented by curve 59 the tap of voltage divider 39 must be displaced to an extent such that the potential of cathode 28 is represented by a line 69. The critical potential of control electrode 29 may then be represented by a line H intersecting curve 68 at point G, and the control electrode releases the flow of current through anode 26 at the same moment as in the absence of filter 5|, 52. The critical potential of control electrode 3| may be represented by a line 12 intersecting curve 68 at point H. Instead of being delayed until the end of the overlap period at point C, the initiation of the flow of current through anode 21 is then obtained at point H substantially simultaneously with the initiation of the fiow of current through anode 26. At point H the voltage between anodes 26 and 21 is equal to the inductive drop caused in winding 13 by the rising flow of current through anode 26 and this voltage drop forces the flow of current to divide substantially equally between the two anodes. The above sequence of operation is repeated for all pairs of valves 23 with the result that the flow of current is initiated substantially simultaneously through both anodes of each pair of parallel valves. All valves also carry currents of substantially equal values, and the two valves of each pair operate during substantially coextensive periods.

Filter 5|, 52 may also be tuned to resonance for a frequency equal to three times the fundamental operating frequency of circuit 6. The filter then amplifies the third harmonic component, which is always present in small amounts in the voltage of alternating current circuits such as circuit 6, and causes the voltage appearing in Winding 36 to assume a wave form of the type represented by curve 13. Such voltage has a steeper wave front than the sinusoidal voltage represented by curve 68, and the difference in time between the moments of initiation of the flow of current through parallel anodes is thus reduced to the point of being practically imperceptible.

It may also be found advantageous to utilize filters more elaborate than the filter above considered. For example, as illustrated in Fig. 4 the filter may comprise reactors 5| and capacitors 52 cooperating with a three phase bank of series resonant elements connected in shunt between the different connections of the reactors 5| with winding 36. In their simplest form the series resonant elements consist of reactors 1'4, each serially connected with a capacitor 16. The effectiveness of the entire filter at different fre-.

quencies may then be represented by the curve of Fig. The filter possesses two different resonant frequencies of which the higher may be equal to three times the fundamental operating frequency of circuit 5 to cause the voltage of winding 36 to assume the form represented by curve 13. In addition, reactors 14 and capacitors 76 may be tuned to resonance for a frequency equal to twice the operating frequency of circuit 6 to suppress any second harmonic voltage which would be impressed on winding 38 as a result of the flow of second harmonic currents through resistors 43.

Although but two embodiments of the present invention have been illustrated and described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.

It is claimed and desired to secure by Letters Patent:

1. In an electric translating system, the combination with an alternating current circuit, a direct current circuit, and a plurality of electric valves interconnecting said circuits for the flow of current therethrough between said circuits, of means for controlling the conductivity of said valves in such sense as to cause substantially simultaneous initiation of said flow of current through said valves comprising a plurality of control electrodes severally associated with said valves, means comprising a control transformer connecting said direct current circuit with said control electrodes for impressing on the latter a potential of such sign and magnitude as to allow the initiation of the flow of current through said valves, and means comprising an electric filter connecting said alternating current circuit with said transformer and operable to cause a component of predetermined wave form of the voltage of said alternating current circuit to be 1m pressed on said transformer and therethrough on said control electrodes.

2. In an electric translating system compr1sing an electric current supply circuit, current output circuit, one of said circuits being an alternating current circuit and the other one of said circuits being a direct current circuit, and a plurality of electric valves connected with said circuits for the flow of current therebetween through said valves, the combination with means for controlling the conductivity of said valves comprising a control electrode in each of said valves, connections between said control electrodes and said direct current circuit comprising a control transformer, and connections between said control transformer and said alternating current circuit, of means for causing substantially simultaneous initiation of the current flows through said valves comprising an electric filter of the low pass type in one of the second said electrodes, said connections comprising a portion of impedance causing the potential impressed from said source on said control electrodes to comprise alternating components of frequencies other than said fundamental frequency, of means for causing substantially simultaneous initiation of the current flows through said valves comprising electric filter means in said connections at a point connected with said source through substantially all of said impedance portion.

4. In an electric translating system, the combination with a plurality of electric valves connected in parallel circuits, means for controlling the conductivity of said valves comprising a control electrode in each of said valves, a source of substantially sinusoidal alternating voltage of predetermined fundamental frequency, and con nections between said source and said control electrodes, said connections comprising a resistive portion causing the potential impressed from said soiu'ce on said control electrode to be distorted from the sinusoidal form, of means for causing substantially simultaneous initiation of the current flows through said valves comprising electric filter means in said connections at a point connected with said source through said resistive portion.

HAROLD WINOGRAD. 

